LIGHTSHIPS OF THE NORTH SEA AND BALTIC SEA 



Because oE" the higher latitude locations of the North and Baltic Seas, 

 the North Atlantic storm tracks influence these sea regions throughout the 

 year. Storm activity here is more frequent and more intense during the 

 autumn and winter seasons. Therefore, a higher number of occasions above 

 and below certain threshold heights are experienced at the North and Baltic 

 Sea lightships in autatin and winter. The spring and summer seasons are 

 controlled largely by the Azores high pressure circulation, which produces 

 long periods of favorable wave conditions. These points are reflected in 

 the persistence curves for the North and Baltic Sea lightships in Figures 

 7A through 7F 



Although the autumn and printer storms are of severe intensity, the 

 buildup of waves to higher height categories and unfavorable wave condi- 

 tions is hindered both by the shallow waters of the North and Baltic Seas 

 and by the limitations of their semienclosed and closed basins to fetch 

 distances. The shallow waters retard wave growth (both in height and 

 length) within the basins, and wave shoaling (the alteration of a wave 

 proceeding from deep water into shallow water) reduces much of the energy 

 content of waves through bottom friction and thus shortens wavelengths 

 and steepens wave crests often to the point of breaking far offshore. 

 Shallowest depths are present at Elbe I Lightship in the North Sea. How- 

 ever, the sheltering effect of land on oceanic wave trains headed toward 

 these sea regions and the rapid migration of storms through these regions 

 probably are more responsible for the short durations of unfavorable waves 

 at the selected lightships. Maximum sheltering occurs at the Baltic Sea 

 lightships (Figures 7E and 7F). 



On occasion, however, when winds of gale force are blowing steadily 

 onshore, water regions of shoaling bottom are plagued by the development 

 of steep crested seas on the far end of fetch areas. The occurrence of 

 this phenomenon on the Danish side of the North Sea during a western gale 

 is notoriously dangerous to small ships, as well as the western part of 

 the English Channel when storm waves are directed from the open Atlantic. 



Persistence of strong onshore winds over a period of time piles up 

 the water at the far end of the fetch area along the shoreface. When a 

 basin lacks an outlet for sufficient drainage, the result is a storm surge 

 or seiche. Storm surges raise the normal level of the water and permit 

 waves to n;n farther up the shoreface before breaking. When a strong 

 surge from a meteorological influence occurs in coincidence with the 

 period of high tide, both reaching their peak at the same time, the result 

 is a flooding of the adjacent lowlands and possible damage to breakwaters 

 and seawalls by the tremendous force of pounding breakers. It was this 

 type of situation which brought about the disastrous North Sea storm of 

 February 1953 which inundated over 1,U00 square miles in Holland and 

 England, claimed l,6ll lives, left 100,000 people homeless, and caused 

 untold millions of dollars of damage to property and livestock. 



52 



